Magnetic head for perpendicular recording



Patented June 1, 1954 UNITED STATES PATENT OFFICE MAGNETIC HEAD FORPERPENDICUIV'AR RECORDING Ragnar Thorensen, Baldwinsville, N. Y.,assignor to General Electric Company, a corporation of New YorApplication June 9, 1951, Serial No. 230,775

face of the recording medium. Accordingly, the

tWo types are called longitudinal and perpendicular recordingrespectively. In most complayback, as well as greater storage per unitarea of the recording medium.

It is an object of my invention to provide a new and improved magnetichead for recording of the perpendicular magnetic type.

It is a further object of my invention to provide a magnetic head forperpendicular recording which makes possible a more directionalmagnetizing field and better resolution.

It is a still further object of my invention to provide a magnetic headfor perpendicular reputers.

And it is yet another object of my invention to provide a magnetic headfor perpendicular recording which has very low inductance, permittingrapidly changing or high frequency signal current to be passedtherethrough for faster recording.

In carrying out the foregoing objects of my invention I provide twoseries-connected parallel portions augment each other. The conductorportions are relatively closely spaced with re spect to each other andalso closely spaced With 8 Claims. (Cl. 1779-1002,)

respect to the surface of a recording medium,

and in close proximity to a recording medium. The loop may be supportedon a block of insulatplacements as produced by the prior art type headof Fig. 4.

Referring now to Fig. l, one embodiment of a magnetic head l of thepresent invention is illustrated in detail, to a large scale, as appliedin a magneticv tape recording system of the perpendicular type. Magnetichead l comprises two parallel conductor portions 2 and 3 connectedserially, preferably formed, as shown, by a single conductor 4 in loopconfiguration with the end sections of the loop turned up and away atsubstantially right angles from the plane oi conductor portions 2 and 3.The series loop combination of conductor portions '2 and 3 is providedwith two terminals and G for connection to a signal current supplycircuit, not shown, the output signals or which are to be recordedmagnetically. Conversely, when head l is employed as a reproducing meansfor signals previously magnetically recorded, terminals 5 and areconnected tc a suitable amplifying and output utilization circuit, alsonot shown. A mounting block l, made preferably of a nonconductive highlypermeable material, such as ferrite, or made of any suitable nonmagneticinsulating material such as plastic or synthetic resin, supportsconductor l With conductor portions 2 and 3 in spaced parallel relation,and preferably closely spaced relation. Terminals 5 and 6, to which theends of conductor Il are connected, may be suitably fastened into blockl, as shown, with the extremities of conductor 4 being bent essentiallyat right angles to conductor portions 2. and 3, which are positioned ingrooves and d provided in the lower face of block l. A retainingprojection H3 on the end of block i opposite terminals 5 and serves tosupport the end of the loop formed by conductor 4, preferably with theend-loop section of conductor i bent essentially at right angles toportions 2 and 3 as shown. I prefer to make grooves S and 9 relativelyshallovv in depth so that conductor portions 2 and 3 project therefromand to mount block 'l by any suitable means such as a bracket, notshown, so that portions 2 and 3 have a small clearance, of the order of.002 to .003 inch, from the surface of the recording medium. In thisway, neither portions 2 and 3 nor block i contact the moving recordingmedium, causing wear thereof, and at the same time portions 2. and 3 maybe closely spaced to the surface o the recording medium. However, if itis preferred, grooves 8 and S may be made somewhat deeper in order thatportions 2 and 3 are flush with the lower surface of block i, and theblock 1 allowed to ride in intimate contact with the recording medium.

As illustrated in Fig. l, the recording head i may be used forperpendicular recording on a tape H which is a moving recording medium.Tape il may be of any form well-known to those skilled in the art oimagnetic tape recording. It may be a homogeneous metallic tape havingrelatively great retentivity and coercive force so that it retainsmagnetism induced therein as it moves past head i; or it may include, asshown, a base portion l2 of a suitable material such as paper or plasticcovered with a layer i3 or relatively great retentivity and coerciveforce material, such as powdered iron oxide. In order to aid thepenetration of the magnetic field delivered by head l, when excited,through tape il, a highly permeable backing member i3, made of ferrite,Permalloy, mumetal, or materials of similar nature, and laminated toprevent excessive eddy currents therein, is positioned on the other sideof tape Il opposite head i. Tape li may ride on backing member i4 ifundue attrition to the tape or backing member does not result, or a verysmall clearance gap between tape il and backing member i4 may beprovided.

In Fig. 2, a plurality of magnetic heads l are shown in a perpendiculardrum magnetic recording system. The moving recording medium in this caseis a cylindrical member or drum l5, which includes a highly permeablebacking portion i6 made preferably of ferrite, Permalloy, mumetal, ormaterial oi like nature formed ci axially stacked laminations tosuppress eddy currents and covered with a layer Il oi relatively greatretentivity and coercive force material such as powdered iron oxide.Heads i are preferably supported with a small clearance, in the order of.O02 to .003 inch, from the surface of layer il by suitable means suchas brackets, (not shown) although, as explained for Fig. l, they may bemade to ride in intimate contact with the surface of drum l5, ifdesired. As illustrated in Fig. 2 it is often desirable, in either tapeor drum recording, to employ several heads spaced closely side by sidein order to record information in parallel tracks on the recordingmedium. The ends of the loop are, therefore, bent upwardly fromconductor portions 2 and 3 and from the recording medium surface toprevent cross interference between adjacent tracks and provide arecorded track sharply denned at its edges. The flux produced by theupturned ends of the conductor loop during recording by heads l islongitudinal to the recording medium surface and is thus entirelyineffective in causing perpendicular magnetism in the recording medium.

In recording operation, with either tape H or drum I's, the aidingmagnetic elds, surrounding conductor portions 2 and 3, due tomagnetizing signal current supplied to terminals 5 and 6, and flowingthrough conductor li, penetrate the high retentivity and coercive forcelayers by being attracted perpendicularly into (or out of) the permeablebacking material. The return path of the flux is a random one throughthe surrounding air space and backing material, the flux density beingso reduced return path as to be ineiective in causing any permanentmagnetization. As the recording medium moves past head I and the signalcurrent through conductor d varies, a perpendicular magnetic fieldpattern representing the signal current variations is induced in andretained by the high retentivity and coercive force layer. This layer,after recording, may thus be considered as made up of a series ofperpendicularly oriented bar magnets oi infinitesimal width placed sideby side along the direction of travel, the iield strength andpolarization of each such bar magnet being proportional to signalcurrent strength at the instant the magnet passed under the recordinghead, providing the layer is operated on a linear portion of itsmagnetization characteristie. Any of the well known biasing systems anderasing heads may be used in conjunction with head I in a completerecording system.

In playback operation, with either tape l. or drum l5, the movingsurface flux from the recording medium cuts conductor portions 2 and 3and induces in conductor l a voltage proportional to the time derivativeof the flux variation, which at constant velocity of the recording.inediurn, is proportional to the space derivative o the ux variationalong the surface of the recording medium. The voltage variationsapnearing at terminals '5 and 6 may thus be applied to an amplier, ifnecessary, and thence to some utilization circuit.

in the wide random The particular advantages of the loop type head oithe present invention may best be seen by reference to Figs. 3, 4, and6, in which the present invention is analytically compared with thegeneral type of perpendicular recording head used heretofore. In Fig. 3,the magnetic head of the present invention is represented by a sectionalview of conductor portions 2 and 3, separated from each other by thedistance D and from the recording medium by the gap g. The recordingmedium is represented as a high retentivity and coercive force layer I8,corresponding to layers i3 and il in Figs. 1 and 2, backed by highparmen ability portion I9, corresponding to member l 1i and backingportion i5, in Figs. 1 and 2. For a given current flowing in oppositedirections in portions 2 and 3, the resulting magnetic iield and iiuxmay be represented by lines as shown, the permeability of portion I9being taken into account in constructing the flux plot by employing themethod of images, i. e., drawing in images 2 and 3' symmetricallylocated from portions 2 and 3, and making the field around eachconductor portion and its image symmetrical. The concentration of theiiux lines 20 midway between conductors 2 and 3 is apparentfrom Fig. 3,and will be further pointed out in connection with Fig. 5.

In Fig. 4, a conventional type perpendicular recording head il isillustrated comprising basically a highly permeable thin strip polepiece 2?. around which there is wound a coil 23. Coil 23 is providedwith terminals 2li and 25 for connection to signal current producingapparatus or to output amplification and utilization apparatus, notshown, during recording and reproduction respectively. Head 2| isessentially of the type described in United States Patents2,532,808-Faus and 2,361,753-Eilenberger, and in various publications onmagnetic recording. With minor modifications not changing the principleof its operation, it has heretofore been considered and used as the besttype perpendicular recording head and it therefore is classed as priorart with respect to the present invention. The particular advantages andnovel features of the present invention over the prior art type ofmagnetic head 2i will become apparent from the following paragraphs.

I-iead 2i is illustrated in conjunction with a recording medium having alayer i8 and high permeability portion Li identical to those of Fig. 3,being similarly separated from the recording medium by a gap g. For agiven current flowing in coil 23, it is known that the resultingmagnetic ux and field may be represented as shown by lines 26.

It is to be understood that the magnetic head of the present inventionmay be used with ad vantage in recording and reproducing sound, such asspeech and music. However, it finds special and more urgently neededapplication in magnetic storage-components of binary digital computers,wherein a series of pulse signals must be rapidly recorded on andreproduced from a relatively small area of the recording medium. Inorder for the pulses to be rapidly recorded and reproduced, it isnecessary that the recording medium surface move rapidly past therecording or reproducing head, for example, at speeds in the order of4000 inches per second. Any contact ci the head with a surface moving atsuch speeds would most certainly cause damaging wear to either the heador the surface. For these reasons, the heads and recording mediumsurfaces in Figs.

3 and 4 have been shown as being separated by a clearance gap g and acomparison will be made on the basis of such a clearance gap. Theclearance gap y, however, should 1n practice be kept as small asconsistent with the mechanical accuracy of the recording medium and headsupporting structures, and any vibrations likely to occur therein, thegap being closed entirely in cases where attrition is not excessive as aresult.

Referring now to Fig. 5, I have shown a plot of the flux densityoccurring at the surface of the recording medium beneath the magnetichead of my invention for a given current flowing in conductor 4. 'I'heordinates of the curves are expressed as a percentage of the maximumsurface flux density which is located midway between portions 2 and 3;and the abscissae of the curve are expressed in terms of the gapdimension g for displacements measured to the left and right of themidpoint between portions 2 and 3 along the direction of recordingmedium movement. It may be shown that the iiux density along the surfaceof the medium is proportional to l/ar2 where x is distance from thecenter of the head. Curve 2l illustrates such a flux densitydistribution along the surface of the recording medium for magnetic headI, in which the distance D which separates portions 2 and 3 is equal totwice the gap dimension g. Curve 28 is the same except that D is equalto Q'. Notice that in both instances the curves are very sharply peakedat the center, indicating that the flux is considerably concentrated. Ifthe magnetic spot size be deiined as the distance between 50% points onthe curve, the spot size for curve 2"! is 1.8 times the gap dimension,or 1.89, and the spot size for curve 2t is 1.2 times the gap dimension,or 1.29. It is practical to consider a clearance gap g of .003 inch anda separation distance D of 2g, or .006 inch. With these dimensions, thespot size would be approximately .005 inch; and if the distance D werey, the spot size would be approximately .004 inch.

ln Fig. 6, curve 2S illustrates the iiux density variation curve forhead 2l of conventional construction with a given current flowing incoil 23. It may be shown analytically that the surface flux density atdisplacements s: from the center of the head falls olf as l/x. In amanner similar to that of Fig. 5, the ordinates of curve 20 areexpressed as a percentage of the maximum flux density which occursdirectly under pole piece 22, and the abscissae are expressed in termsof the gap dimension g as displacements from pole piece 22 along therecording medium surface. It will be seen that curve 2Q is much lesspeaked than curves 21 and 28, indicating that head i is more directiveand a denite improvement over the prior head 2|. The spot size for head2l is seen to be 3.7 times the gap dimension, or 3.7 g. Assuming again apractical gap dimension oi .G03 inch, the magnetic spot size for head 2lis .011 inch, or roughly twice as large as the spot size indicated forcurve 2l. The resolution of the magnetic head of the present inventionis, therefore, approximately twice as great as that of previousperpendicular recording heads.

The inductance of conductor Ll in loop con guration is very low since itforms a coil of only one turn, and the current therethrough may bechanged rapidly in response to rapidly changing applied voltage.Further, there can be no eddy current energy losses in the head if block'l is made of insulating material. To reduce the current required for agiven magnetizing ilux dening surface of said recording medium with theplane of said conductor portions parallel to the plane of said surfaceand the directions of current flow through said conductor portionsperpendicular to the direction of motion of said surface.

6. In a perpendicular magnetic recording and reproducing system, amagnetic head comprising a mounting block, a conductor mounted inelongated loop conguration on said block, terminals on said blockconnected to said conductor to conduct a signal current therethrough,said block having two grooves in parallel spaced relation in one facethereof, and said conductor having two conductor portions disposed insaid grooves and the end sections of the loop formed by said conductorbent at essentially right angles to the plane of said portions, wherebythe magnetic elds produced around said portions by said signal currentaugment each other in the space between said portions in a directionperpendicular to the plane of said portions.

7. A magnetic head comprising a single conductor arranged in elongatedloop configuration, means to support said loop parallel to the surfaceof a magnetizable medium, terminals connected to said conductor to applya signal current thereto, said conductor having the end sections of theloop formed by said conductor bent at essentially right angles to theplane of said loop, whereby the magnetic elds produced around said endsections are eiectively eliminated from the fields produced around saidloop.

8,. In a magnetic pickup and recording device, a lamentary conductoradapted to be supported in a continuous loop and to conduct a signalcurrent therethrough, the plane of said loop being parallel to that of amagnetizable medium, and a mounting member for said conductor, saidmember being of a high-permeability, high-resistivity material, wherebyto provide increased magnetic fleld strengths for a given signalcurmember.

References Cited in the file of this patent UNITED STATES PATENTS

